The present invention concerns an apparatus for embossing thermoplastic web material and a method of employing the apparatus to provide an embossed material, preferably a material embossed with two overlying patterns.
Use of a pair of embossing rolls arranged to move relative to each other to form a roll nip therebetween is of course old and well known in the art. Similarly, the use of an embossing roll having a resilient outer surface providing an embossing pattern is also known and, although the prior art generally employs a combination of one resiliently faced roll and one rigid roll such as a conventional steel roll, the use of a pair of resiliently faced rolls is known. For example, see U.S. Pat. No. 3,605,194 issued Sept. 20, 1971, Jan P. Nauta. See also, U.S. Pat. Nos. 3,636,147; 3,751,550; 3,839,514; and 3,950,480 dealing with embossing roll apparatus.
Certain difficulties are encountered with prior art apparatus and methods when it is attempted to emboss relatively thin web material of up to about 0.050 inches (fifty mils) thick, with patterns of which are of the depth as great or greater than the thickness of the web material.
One such difficulty occurs when it is attempted to emboss a relatively deep pattern into such a relatively thin thermoplastic sheet by conforming the heated material to the embossing pattern. For example, the thermoplastic sheet of material 3 mils thickness may be embossed with a pattern up to fifty mils in depth by stretching and pressing the sheet material into conformity with an embossing pattern having pattern recesses of about 50 mils in depth and cooling to set the pattern. There is a tendency for the plastic sheet material not to conform fully to each bond and recess provided by the embossing pattern. The difficulty is aggravated by the entrapment of air between the material and the roll pattern, the entrapped air pockets tending to hold the sheet material out of such surface conforming contact. Vacuum forming of individual lengths of sheet material overcomes this problem but is not adaptable to a continuous web embossing process.
Yet another difficulty is encountered when it is attempted to simultaneously emboss two overlying patterns on the sheet, one of which is a deep pattern relative to the thickness of the film. Heretofore, it has been difficult to superimpose the second pattern fully onto the deep pattern because of the inability of the roll providing the second embossing pattern to emboss in the deeper recesses of the deep pattern.
It is accordingly an object of the present invention to provide a novel method and apparatus for embossing web material which overcomes the foregoing and other difficulties.
It is another object to provide such a novel embossing apparatus utilizing first and second embossing members, each employing a resilient embossing surface of a selected range of hardness, with one embossing surface being harder by a specified amount than the other and having a primary embossing pattern thereon.
Other objects and advantages of the present invention will become apparent from the following description thereof.